In some applications, it is necessary to determine the temperature of a workpiece whose temperature is difficult to directly measure. For example, where the workpiece is a semiconductor wafer, although the temperature of a substrate side of the wafer may often be measured directly, it is not possible to accurately directly measure the temperature of the device side of the wafer, as the non-uniform patterning of devices on the device side gives rise to significant variations in both scattering and emissivity from location to location across the device side, resulting in significant temperature measurement errors.
In the past, this did not pose a significant problem, as many rapid thermal processing cycles involved heating the substrate side of the wafer at heating or ramp rates that were slow compared to a thermal conduction time through the wafer (typically 10-15 milliseconds), so that the temperature of a given location on the device side could be presumed to equal the temperature of the corresponding opposing location on the substrate side, and the errors resulting from such a presumption were not critical for the purpose of achieving performance requirements applicable at that time (which are now quickly becoming obsolete).
However, these conventional techniques cannot produce sufficiently shallow junctions to comply with current and upcoming industry requirements. A new technique to address this difficulty is disclosed in pending Patent Cooperation Treaty application publication numbers WO 02/47143 and WO 02/47123 (which are incorporated herein by reference), and involves pre-heating the entire wafer to an intermediate temperature by irradiating the substrate side at a ramp rate slower than the thermal conduction rate through the wafer, then heating the device side of the wafer at a rate much faster than the thermal conduction rate, by irradiating the device side. As an arbitrary example, the wafer may be pre-heated to an intermediate temperature such as 800° C. for example, by irradiating the substrate side with an arc lamp to heat the entire wafer at a rate such as 400° C./second, for example. The device side may then be exposed to a high-intensity flash from a flash lamp, such as a one-millisecond flash, to heat only the device side to an annealing temperature such as 1300° C., for example. Due to the rapid heating rate of the device side during the flash (on the order of 105° C./s), the bulk of the wafer remains at the intermediate temperature, and acts as a heat sink to then cool the device side following the flash.
To minimize performance variations from wafer to wafer, it is important to ensure that each wafer is subjected to a consistently reproducible thermal process, as close as possible to an identical process from wafer to wafer. For this purpose, it would be desirable to accurately measure the temperature of the device side of the wafer during the flash, and to use such temperature measurements for feedback-control of the intensity of the flash. However, as noted, conventional methods are not adequate to accurately measure the temperature of the device side for this purpose.
Accordingly, there is a need for an improved way of measuring the temperature of a workpiece, and for an improved way of heat-treating the workpiece.